1. Field of the Invention
The present invention relates to a sound pick-up and reproduction system provided with a device for reducing the echo resulting from the acoustic coupling between its sound pick-up and its reproduction device.
An acoustic system according to an embodiment of the present invention can, for example, be a system for picking up and reproducing sound consisting of a sound pick-up for picking up sound, such as a microphone, a reproduction device for reproducing the sound, such as a loudspeaker, and an application unit, such as an amplifier, to increase the power thereof in order to transmit it to the reproduction device.
It can also be a telephone unit also including a sound pickup for picking up sound such as a microphone, an application unit which will send the signal originating from the sound pickup to a distant telephone unit and a reproduction device provided for reproducing the signal originating from the distant unit.
In such systems, the signal emitted by the reproduction device can be picked up by the sound pickup, and an acoustic echo results therefrom. This acoustic echo can be broken down into two main parts: the loudspeaker/microphone direct coupling and the response of the room.
2. Description of the Related Art
Although the response of a room is in general dealt with effectively by conventional echo cancelling techniques (on account, among other things, of the fairly low level of this part of the echo), the same does not apply for the direct coupling, which results both from the acoustic mode and from propagation of the vibrations, for example through the case if the system is a telephone terminal. This part of the echo due to the direct coupling has in general a higher power than the local speech picked up by the sound pick-up (by 6 to 18 dB, that is a multiplying coefficient of 8), and brings about a self-oscillation phenomenon generally referred to as the xe2x80x9cLarsen effectxe2x80x9d if no suitable processing is applied.
Although echo cancelling makes it possible to deal with this harmful effect to a greater or lesser degree, a consequent variation in gain must be introduced in addition in order to avoid onset of the Larsen effect during the convergence phase of the echo canceller, and also later to prevent any problem if the echo path varies or in times of duplex speech.
For a telephone terminal, the direct acoustic coupling is large inasmuch as the microphone is placed in the same case as the loudspeaker. The echo which results from this coupling is the major obstacle to good operation of hands-free telephones.
It has therefore been sought to reduce the effects of the direct coupling. The microphone/loudspeaker distance cannot be changed in significant proportions in view of the size of the individual terminals. It should be noted however that, each time the distance is doubled, the power of the echo is divided by four.
It is on the other hand possible to act on the mechanics of the coupling between the loudspeaker and the microphone. For example, the microphone can be dissociated from the case using antivibratory materials, such as foams or rubber, which has the effect of removing or greatly decreasing the vibrations generated by the loudspeaker and transmitted, via the case, to the microphone.
This mechanical decoupling solution effectively decreases the vibrational coupling, but proves to be an industrially expensive solution. Furthermore, it does not reduce the acoustic coupling which can prove to be significant if the microphone is placed relatively close to the loudspeaker, which is the tendency today on account of the fashion for small-sized terminals.
Another known solution consists of using, in parallel with the echo signal path, a compensating filter whose response is the inverse of the pulse response of the coupling between the microphone and the loudspeaker, a coupling both mechanical and acoustic. In theory, at the output of the compensating filter, the signal coming from the loudspeaker due to the echo is nullified. However, in practice, this technique is fairly specialized and no longer gives satisfaction if the coupling has changed in its characteristics, even in a trivial way, following, for example, disassembly/reassembly of the terminal. This technique is furthermore not suitable if the nature of the perturbations is not linear, that is to say if they cannot be modelled by the product of the signal coming from the loudspeaker and a linear function. What is more, the operation of transducers is seldom linear, since they are generally subject to distortions and/or saturations, which are typical examples of non-linear functions.
Finally, for mass-production, the coupling will inevitably be different from one terminal to another, on account for example of the use of a slightly different loudspeaker or microphone, there again making this filter compensation technique not very effective.
The aim of the present invention is therefore to propose an acoustic system provided for reducing the echo which results from the direct acoustic coupling between its sound pick-up and its reproduction device which makes it possible to solve the problems mentioned above.
To that end, a sound pick-up and reproduction system according to an embodiment of the present invention includes the sound pick-up having at least two sound sensors situated at different distances from the reproduction device and a processing unit provided for using the amplitude and phase of the signals originating from each sound sensor in order to deliver a signal to the application unit whose echo signal has been reduced.
According to another aspect of the present invention, the processing unit performs filtering of the signal originating from each sensor, and then a summation of the signals thus filtered, the pulse response of each filtering associated with a sensor being defined by a function gi(t) which is a solution of the following equation:
xcexa3[hi(t)]* gi(t)]N 0 for i varying from 1 to n
where hi(t) (i varying from 1 to n) is the pulse response of each sensor to the signals delivered by the reproduction device.
According to another aspect of the present invention, one of the filterings is defined by a multiplication function.
According to another aspect of the present invention, the pulse response of each sensor to the signals delivered by the reproduction device is determined by measuring the response of the said sensor to signals emitted by the reproduction device.
According to another characteristic of the present invention, each function gi(t) is determined by solving the said equation by means of an adaptive algorithm.
According to a variant embodiment of the present invention, each function gi(t) is determined by solving the equation with a mean square algorithm.
According to another aspect of the present invention, the system has two sensors, one of which is closer to the reproduction device than the other.
According to another aspect of the present invention, the filtering associated with the sensor which is the further from the reproduction device is defined by a multiplication function.